As is well known, refrigerating units, such as those used for refrigeration or air conditioning, are comprised of a compressor that compresses the refrigerant vapor. The vapor is then cooled and condensed by means of contacting with a so-called "hot source" (less hot than the gas coming from the compressor) and the pressure of the condensed fluid is then decreased by means of an expansion valve down to a pressure low enough to enable the vaporization of the fluid which then comes into contact with the so-called "cold source." Thereafter, the vaporized gas is returned to the compressor. During its vaporization, the fluid absorbs heat from the cold source, thus creating the desired refrigerating effect.
This type of unit needs to be controlled so as to avoid two drawbacks. Firstly, if the amount of heat available at the cold sources is low, large quantities of residual liquid may be returned to the compressor. The result of this would be damage to the compressor and waste of energy in the unit. Secondly, if the amount of heat available at the cold source is excessive, the rate of the liquid flow arriving at the evaporator may not be high enough to maintain the cold source at the desired low temperature level. When the amount of heat available at the cold source is excessive, the motor of the compressor would be damaged because of overloading.
In order to achieve the required regulation or control, it is known that the evaporator is controlled by the use of a thermal expansion valve with a bulb partially filled with liquid (generally the same liquid as the fluid used in the refrigerating circuit).
The use of the thermal expansion valve cannot provide safe utilization of the unit when the amount of heat available at the cold source is excessive. When used in this scenario, the thermal expansion valve promotes substantial energy losses. An attempt to overcome this drawback was made in the U.S. Pat. No. 5,195,331 by Zimmern et al, Mar. 23, 1993. According to this invention, the bubble of the expansion valve is heated by a resistor and mounted in the discharge pipe of the evaporator. In the absence of droplets of liquid refrigerant in the flow through the discharge pipe, e.g. when the refrigerant flow rate tends to become too low with respect to the cold demand, the resistor heats up the fluid in the bulb, and the pressure in the bulb increases and moves the expansion valve to a more opened position. As soon as droplets hit the bulb in the discharge pipe, said droplets cool down the bulb despite the heating effect of the resistor and the expansion valve is moved to a more closed position. The disadvantage of this solution is that it does not allow proper working of the refrigerant unit when the amount of heat available at the cold source is excessive, and in this situation, it promotes substantial energy losses.
In conclusion, a need exists for improving the refrigeration unit when the amount of heat available at the cold source is excessive.